APPLYING GEOMETRIC MORPHOMETRIC ANALYSIS TO ASSESS MORPHOLOGICAL AND ONTOGENETIC VARIATION IN CLYPEASTEROID ECHINOIDS; AN EXAMINATION OF ARACHNOIDES PLACENTA

The irregular echinoid order Clypeasteroidea originated during the Late Paleocene, and is currently globally distributed. During this short interval Clypeasteroides, or sand dollars, obtained surprising levels of morphological diversity, with body forms optimized for their specific niches. The group is typified by a flattened appearance that is highly derived from the ancestral morphology of strongly bilaterally symmetrical irregular echinoids. Though varied, clypeasteroid morphological disparity is most readily apparent in variation of the body outline or ambitus. Body forms cover a broad range, including distinctive perimeters; elongate forms; inflated body chambers; and pentagonal, triangular, to circular outlines. Interestingly, disparate clypeasteroid clades obtained circular morphologies independently. This analysis develops a framework to quantify how a distinctive circular ambitus is obtained by some clypeasteroids by examining the extant species Arachnoides placenta. This study utilizes historical collections from the National Taiwan University; a data set consisting of 26 individuals ranging from small juveniles to adults morphotypes. Geometric morphometric analysis is used to quantify the ambitus and interior petaloid morphology of A. placenta through digitization of landmark and semi-landmark points.

The goals of quantifying morphological variation in Arachnoides placenta are to understand: 1) How a circular ambitus developed in the genus and how this morphology may be obtained in other clypeasteroids; 2) To evaluate developmental variability within clypeasteroids by assessing how ontogenetic development varies in the A. placenta study system; 3) How the strongly bilateral irregular echinoid ancestral morphology approaches a circular outline in disparate clypeasteroid clades by using A. placenta as a frame work. Results from the A. placenta data set suggests a circular adult morphology derives from asymmetric ambitus change, requiring allometric shape change during ontogeny. This research provides the required background to quantitatively assess ontogenetic and morphological variation in both fossil and extant clypeasteroid data sets in the future.